High-carbon ferrochromium (hereinafter simply referred to as “ferrochromium”) is manufactured by smelting and reducing chromium ore after pretreatment in a submerged arc electric furnace (hereinafter simply referred to as “electric furnace”). Examples of the pretreatment of the chromium ore include briquetting, sintering, pellet firing, and pellet prereduction.
In pellet prereduction, chromium ore is pulverized with coke and is granulated to prepare green pellets, which are then subjected to reduction roasting in a rotary kiln at 1,300° C. or higher to provide prereduced pellets. The reduction degree of the prereduced pellets, which is 60% to 70% only with the internally added coke, reaches 80% in combination with externally added coke. This method therefore has a significantly smaller amount of heat required for the reduction of chromium ore in an electric furnace than other types of pretreatment, thus greatly reducing power consumption.
Pellet prereduction is an excellent method with low power consumption; however, this method, involving the use of a rotary kiln for the pretreatment, has the following many problems unique to the rotary kiln. Because the fundamental principle of the rotary kiln is based on the tumbling of feedstock, the rotary kiln disadvantageously produces a large amount of dust to readily cause dam rings therein. In addition, the rotary kiln requires an excessive length due to variations in the residence time of the feedstock, thus involving a large equipment installation area and a large surface area. Consequently, the rotary kiln disadvantageously dissipates a large amount of heat, leading to high fuel consumption. Furthermore, a combination with externally added coke is disadvantageous in that it causes a large oxidation loss of the externally added coke in the rotary kiln.
Chromium oxide is reduced less easily than iron oxide from a thermodynamic point of view. The temperature of the pellets in the kiln is gradually raised by heating the pellets with a burner provided on a discharge end of the kiln. Accordingly, the internally added coke is consumed preferentially in the reduction of iron oxide contained in the chromium ore since iron oxide is reduced more easily than chromium oxide. As a result, the reduction of chromium oxide lags behind since chromium oxide is reduced less easily than iron oxide.
To solve the above problems unique to rotary kilns, methods are proposed in which a rotary hearth furnace is used for the prereduction.
In one method, green pellets prepared by adding a carbonaceous material to a steel mill waste containing Cr and Fe and granulating the mixture are preheated to about 600° C. to 800° C. with a shaft preheater, are charged into a rotary hearth furnace, and are gradually heated to about 1,000° C. to 1,800° C. in a reducing atmosphere.
In another method, green pellets prepared by adding a proper amount of chromium ore to a chromium-containing waste produced in the manufacturing process of stainless steel and granulating the mixture with coke are placed on a hearth of a rotary hearth furnace and are heated with a combustion gas to manufacture pellets containing chromium and iron.
The above methods, in contrast to rotary kilns, produce a less amount of dust and therefore cause no dam ring because the feedstock placed on the rotary hearth is stationary. In addition, no excessive hearth area is required since the residence time of the feedstock is uniform. Accordingly, the equipment used is more compact and the furnace surface area is smaller, so that the furnace has a less amount of heat dissipated to provide lower fuel consumption.
In the above methods, however, the internally added carbonaceous material starts to reduce iron oxide even at about 600° C. to 800° C. in the shaft preheater (while the carbonaceous material does not reduce chromium oxide at such temperatures). In addition, the pellets are gradually heated in the rotary hearth furnace; as a result, the carbonaceous material is consumed preferentially in the reduction of iron oxide. By the time the furnace reaches the temperature at which the reduction of chromium oxide can start, the chromium oxide loses the opportunity to come into contact with the carbonaceous material for lack of the carbonaceous material to give a low chromium reduction degree. On the other hand, increasing the amount of carbonaceous material added internally to maintain the contact opportunity causes the following typical problems: the green pellets disintegrate due to a decrease in strength to form deposits on the hearth; the dust loss from the rotary hearth furnace to the flue gas is increased; and the reduced pellets disintegrate, or otherwise their density decreases, to cause difficulty in dissolving in molten metal in an electric furnace, leading to a lower smelting yield.
Furthermore, the above methods make no mention of the heating temperature and temperature raising rate of the pellets and the above problem that the reduction of chromium oxide lags behind.
Accordingly, an object of the present invention is to provide a method for reducing chromium oxide in a chromium-containing material (a method for manufacturing reduced chromium). When a chromium-containing material that contains chromium oxide and iron oxide and is provided with an internally mixed carbonaceous material is reduced (prereduced), the method of the present invention can promote the reduction of chromium oxide while suppressing preferential consumption of the internally added carbonaceous material in the reduction of iron oxide, thereby increasing the chromium reduction degree.